A metal detection apparatus is used to detect and reject unwanted metal contamination. When properly installed and operated, it will help reducing metal contamination and improving food safety. Most modern metal detectors utilise a search head comprising a “balanced coil system”. Detectors of this design are capable of detecting all metal contaminant types including ferrous, nonferrous and stainless steels in a large variety of products such as fresh and frozen products.
The metal detection apparatus typically comprises a metallic enclosure, having entrance and exit apertures with cross-sectional areas of different or equal size defining a travel path inside the enclosure along which an object under inspection moves.
A metal detection apparatus that operates according to the “balanced coil”-principle typically comprises three coils that are wound onto a non-metallic frame or yoke. The coil system comprises at least one transmitter coil and at least one first and at least one second receiver coil; the respective receiver coils bounding a detection zone inside the enclosure between the entrance and exit apertures. The detection zone has a cross-sectional profile that varies or is constant along the travel path. Systems with cylindrical detection zones typically use coils having identical dimensions with the transmitter coil centred between the two receiver coils. Systems with conical detection zones use coils that differ in size from one another typically with the transmitter coil being off-centred between the two receiver coils. In both systems the coils are arranged such that, when the at least one transmitter coil is energized by an alternating electric current, the electromagnetic field generated thereby induces a first voltage in the first receiver coils and a second voltage in the second receiver coil, the first and second voltages cancelling each other out when there is no metal present in the object under inspection.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed. This change in balance results in a signal at the output of the receiver coils that can be amplified, processed and subsequently be used to detect the presence of the metal contamination.
The signal processing channels typically split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coil system. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the product, the contaminants and other disturbances, have different influences on the magnetic field so that the resulting signals differ in phase.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g. a frequency mixer or analogue multiplier circuit, generates a voltage signal which represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants can be detected. However, in the event that the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component.
In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
U.S. Pat. No. 8,587,301 B2 discloses a method for operating a metal detection system that allows determining a preferable transmitter frequency with which signal components of smallest sized metal particles differ most in phase and amplitude from the phase and amplitude of a product signal.
Hence, for testing and optimising a metal detection apparatus tests with different contaminants need to be performed, which is however time-consuming. Furthermore, tuning should be performed regularly particularly in view of the frequent changes of the processed product.
European Patent Application EP15200786.0 discloses a testing device for testing a metal detection apparatus, in which one or more test articles are passed along test axes in parallel or equally inclined to a product axis along which products travel through a detection zone. Each test article is embedded in a slug, which can move freely within a guide tube from one end to the other. Evaluation of the signals induced by the test articles allows checking the sensitivity of the metal detection apparatus.
Transferring the test articles along the test axis, which is offset from the product axis, allows testing the metal detection apparatus while products are transferred. A disadvantage of such a test device is that the sensitivity of the metal detection apparatus is not measured in other regions of the detection zone, which may be more critical than the region, in which the test device is installed.
For different regions of the detection zone which exhibit different sensitivities, the user of the apparatus may however use test articles, which are adapted to the sensitivity in the specific region of the detection zone. A single test article, e.g. as delivered by the manufacturer of the metal detection apparatus however is not suitable for performing tests in regions of the detection zone, which exhibit different sensitivities.